emulsion

A disperse system in which both phases are
liquids, one of which is generally water or an
aqueous solution, and the other an oil or other
water immiscible liquid. The droplets of the
dispersed liquid are known as the inner phase of
the emulsion because it appears to be inside the
liquid medium. The surrounding liquid (the
continuous phase), on the other hand, is called
the external phase. The dispersion may occur
naturally or it may be prepared by mechanical
methods (dripping or slowly pouring one of the
ingredients into the other while stirring
vigorously), or developed as a result of any of
several polymerization processes. The liquid in
the continuous phase is usually water when the
emulsion is designed for pigment coating, as of
paper, for example, or for oil penetration, as in
the FATLIQUORING
of leather. For certain other types of
application, the medium may be an organic liquid.
To form a stable emulsion, a third ingredient must
be present; it is called an emulsifier, or
emulsifying agent, and forms absorbed films around
the tiny globules of the dispersed fluid to
prevent them from coalescing. A common emulsifier
is soap.

A familiar, naturally occurring emulsion is whole
milk, the inner phase of which consists of
globules of butterfat; the external phase is a
watery solution of casein, sugar, and other
substances. Another natural emulsion is egg yolk,
which consists of egg oil in an aqueous solution
containing, among other substances, albumen and
lecithin; the latter is a lipoid (fatlike)
substance that is one of nature's most efficient
emulsifying agents. Albumen, also, is a good
emulsifier.

Milk and egg yolk are oil in water emulsions. A
second type of emulsion is that of water in oil,
in which the phases are reversed. An example of
this type of emulsion is butter, which has aqueous
constituents dispersed in tiny globules throughout
the butterfat. Most manufactured emulsions are
made by combining the oily ingredient with a
colloidal solution, such as casein or albumen;
these are not only good emulsifiers, but also
confer desirable properties when the emulsion is
used as an adhesive.

Emulsions are generally opaque or milky because of
the refraction and dispersion of light by the
minute droplets, but when dry they become
transparent, or nearly so.

In addition to their use in paper and leather
manufacture, emulsions are used in edge gilding,
gold tooling, and other bookbinding operations.
(233 , 235 )